The present invention relates to a spectrophotometer and a method of calibrating the same.
Although various kinds of spectrophotometers are known to perform quantitative analysis and qualitative analysis of a specimen by obtaining an absorbed spectrum of a material contained in the specimen which is a measurement target, as infrared spectrophotometers, there exist, for example, a dispersive infrared spectrophotometer (dispersive IR), a Fourier transform infrared spectrophotometer (FTIR), a non-dispersive infrared spectrophotometer (NDIR), among which the non-dispersive infrared spectrophotometer (NDIR) selects an infrared absorbing wavelength of a specimen by using a bandpass filter with multi-layer films (see JP2000-241346A and JP2005-257358A).
Before an NDIR is shipped out, an initial concentration calibration, that is, a concentration of gas and an infrared light absorption are associated with each other therein. Specifically, first, among NDIRs with the same specification, an instrument body to serve as a reference (hereinafter, may be “reference instrument”) is selected, and with this reference instrument, an equation expressing a relation between an actual gas concentration Cactual—gas and a photo detection signal value x, obtained from a photodetector, is obtained (Cactual—gas=greference—instrument (x)) using the actual gas that is a measurement target. Further, the actual gas is changed to a substitute gas, and an equation expressing a relation between a substitute gas concentration Csubstitute—gas and the photo detection signal value x, obtained from the photodetector, is obtained (Csubstitute—gas=freference—instrument (x)) as well. Thus, based on these equations, a conversion coefficient α with which the substitute gas concentration Csubstitute—gas is converted into the actual gas concentration Cactual—gas is obtained.
Next, with an arbitrary instrument body to be calibrated (hereinafter, may be “calibration instrument”), an equation expressing a relation between the substitute gas concentration Csubstitute—gas and the photo detection signal value x, obtained from the photodetector, is obtained (Csubstitute—gas=fcalibration—instrument (x)) using the substitute gas.
Further, under an assumption in which a ratio in infrared light absorption between the actual gas and the substitute gas (abs.actual—gas/abs.substitute—gas) is constant with any instrument bodies, according to the conversion coefficient α obtained from the reference instrument and the equation expressing the relation between the substitute gas concentration Csubstitute—gas obtained from the calibration instrument and the photo detection signal value x (Csubstitute—gas=freference—instrument (x)), an equation for calculating the actual gas concentration Cactual—gas for the calibration instrument (Cactual—gas=α×fcalibration—instrument (x)) is obtained. Although, with the reference instrument, as described above, C=g(x) is the function for calculating the actual gas concentration and C=f(x) is the function for calculating the substitute gas concentration, in calculating the actual gas concentration in the calibration instrument, C=α×f(x) is used, and the photo detection signal value for a case where the actual gas is adopted for the calibration instrument is substituted as x therein.